The present invention relates to a screwer test bench and in particular to a bench with an innovative brake for simulation of resisting torque.
Screwer test benches are well known in the prior art. Such benches simulate the resisting torque of a screw in tightening and at the same time measure the parameters of torque and angle developed by a screwer applied to the bench and signal deviations of said parameters from a predetermined characteristic curve.
In such benches correct simulation of resisting torque is the most critical part in order that the measurements made be truly representative of the operation of the screwer in normal use.
In accordance with a known system the bench comprises a cap with which is engaged the screwer being tested. A friction disk is thrust against the cap by means of a hydraulic actuator to produce the braking force.
Such known systems suffer from various disadvantages among which the high inertia of the brake and the friction and dragging of the running guides of the movable parts of the brake proved particularly important for the accuracy of bench measurements.
The problem of high inertia is due essentially to the fact that the brake cap is realized of relatively thick metal to support the axial forces developed when the friction disk is thrust against it. The problem of the running guides is due to the fact that all the torque produced by the screwer is unloaded on the friction disk which must therefore be locked against axial rotation. As the thrust cylinder cannot offer any resistance to rotation the disk runs along purposeful guides.
The holder of this application has found that such a structure is adequate for normal brakes whose only purpose is to stop a rotating shaft (for example vehicle brakes) but becomes very disadvantageous in the specific case of screwer measurement benches.
Indeed, the holder of this application has found that, in particular during the last moments before stopping, vibrations are produced which affect the consistency of braking at the most delicate moment of the cycle, i.e. at the moment of peak torque acquisition.
A conventional disk has a relatively high moment of inertia and therefore displays rather low resonance frequency. The standards for torque measurements in screwers require in general however that the measurements be executed with filters which cut only the frequencies above 500 Hz. Because of the moment of inertia, normal brake disks have however a resonance frequency below 500 Hz and therefore all the vibrations of the disk and the shaft to which it is rigidly keyed are transmitted to the torque transducer and the measurement system acquires oscillations in the torque peak which are the main cause of measurement repeatability errors.
The general purpose of the present invention is to remedy the above mentioned shortcomings by making available a screwer test bench having a braking system which reduces inertias and therefore increases resonance frequency. Anther purpose is to make available a bench with a braking system which reduces the problems owing to the need for opposition to the rotation produced by the braking forces.
In view of this purpose it was sought to provide in accordance with the present invention a screwer test bench comprising a coupling designed for connection with the head of a screwer and connected to a controlled braking unit and a sensor unit 15 for detection of mechanical magnitudes transmitted between the coupling and the braking unit characterized in that the braking unit comprises a thin cap constrained to said coupling at least in the direction of rotation and arranged between a first pressure surface thrust axially against the cap by an actuator and a second surface for fixed opposition.